Late metal-silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

The short-lived $^{182}$Hf-$^{182}$W decay system is a powerful chronometer for constraining the timing of metal-silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body. The $\epsilon^{182}$W values obtained for the IAB iron meteorites range from -3.61 $\pm$ 0.10 to -2.73 $\pm$ 0.09. Correlating $\epsilon^{\mathrm{i}}$Pt with $^{182}$W data yields a pre-neutron capture $^{182}$W of -2.90 $\pm$ 0.06. This corresponds to a metal-silicate separation age of 6.0 $\pm$ 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal-silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 $\pm$ 0.1 Ma after CAIs with a radius of greater than 60 km.

💡 Research Summary

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The paper presents a refined chronology for metal‑silicate separation on the IAB iron‑meteorite parent body by combining high‑precision tungsten (W) and platinum (Pt) isotope measurements with thermal‑evolution modeling. The short‑lived ¹⁸²Hf–¹⁸²W decay system is a powerful chronometer for core formation, but its application to iron meteorites is hampered by neutron‑capture effects induced by galactic cosmic‑ray (GCR) exposure, which alter the measured ¹⁸²W isotopic composition. To correct for this bias, the authors employ Pt isotopes as an in‑situ neutron‑dose proxy because Pt and W have similar neutron‑capture cross‑sections. By measuring Pt and W isotopes from the same digestion aliquot for seven IAB samples (spanning the main group, low‑Au low‑Ni, a duo, and an ungrouped specimen), they establish a linear correlation between εᵢPt and ε¹⁸²W. Extrapolation to zero neutron‑capture yields a pre‑exposure ε¹⁸²W value of –2.90 ± 0.06. Compared with the modern CAI reference (ε¹⁸²W = –3.49 ± 0.07), this translates to a metal‑silicate separation age of 6.0 ± 0.8 Ma after CAI formation.

The authors then explore the thermal history of the IAB parent body using 2‑D and 3‑D fluid‑dynamics simulations (I2/I3ELVIS code). The models incorporate heating from the decay of short‑lived radionuclides ²⁶Al and ⁶⁰Fe, latent heat of silicate melting, and a parameterized convective heat flux for melt fractions > 0.4. By varying the initial ²⁶Al/²⁷Al ratio (2.5 × 10⁻⁵ – 0.7 × 10⁻⁵, corresponding to formation times 0.75–2.0 Ma after CAI) and planetesimal radius (20–200 km), the simulations identify a set of conditions that reproduce the observed separation age. The best‑fit scenario requires the parent body to have accreted at 1.4 ± 0.1 Ma after CAI with a radius greater than 60 km, an initial porosity of 0.3, and an ambient temperature of 290 K. Under these conditions, internal heating by ²⁶Al and ⁶⁰Fe raises the interior temperature sufficiently to generate ~4–5 % silicate melt, triggering a global metal‑silicate segregation event at ~6 Ma.

Following this early differentiation, the model predicts a later catastrophic impact between 10 and 14 Ma after CAI that fragmented the body. Subsequent gravitational re‑accumulation mixed metal, silicate, sulfide, and graphite fragments, explaining the heterogeneous trace‑element signatures and the presence of both chondritic and non‑chondritic inclusions observed in IAB irons. This impact‑reassembly scenario reconciles earlier competing hypotheses that invoked either impact‑generated melt pools or purely radiogenic melting.

Key contributions of the study are: (1) validation of Pt isotopes as a reliable neutron‑dose correction tool for non‑magmatic iron meteorites; (2) a tightly constrained metal‑silicate separation age of 6.0 ± 0.8 Ma after CAI, narrowing previous estimates that spanned 3–12 Ma; (3) integration of isotopic data with thermomechanical modeling to infer the parent body’s accretion time, size, and internal thermal evolution. These results advance our understanding of early solar‑system processes, demonstrating that small planetesimals could undergo whole‑body melting driven by short‑lived radionuclides, experience early core formation, and later be reshaped by large impacts, leading to the complex chemical and isotopic diversity observed in the IAB meteorite clan.